Alkali-free accelerators have been added to the spray nozzle of sprayed concrete equipment immediately before the concrete is applied to a rock substrate. Examples are disclosed in WO99/18045, WO05/028398 and WO08/006410. Aqueous sodium silicate, commonly called water glass, has been added to backfill grout shortly before it is pumped into a void. Alkali carbonates have been used to shorten the setting time and to speed up strength development. However, alkali carbonates decompose when added to weakly acidic alkali free accelerators. Addition of carbonate to silicates causes precipitation of silica gel.